The role of cellular morphogenesis in the pathogenicity of the rice blast fungus Magnaporthe oryzae

Appressorium-mediated plant infection is a common strategy used by many plant pathogenic fungi. Understanding the underlying genetic network that controls cellular differentiation of appressorium is therefore pivotal to design durable resistance strategies for these devastating pathogens. This thesis describes four published studies, which investigate the role of septin GTPases in infection and the role of secretion during plant tissue invasion by the rice blast pathogen Magnaporthe oryzae. Appressorium development involves a series of morphogenetic changes that are tightly regulated by cell cycle checkpoints. Entry into mitosis allows differentiation of an appressorium, while penetration peg emergence appears to require progression through subsequent cell cycle checkpoints and cytokinesis. The studies presented here show that symmetry-breaking events that occur during appressorium differentiation are mediated by scaffold proteins, named septins. Septin GTPases recruit actomyosin ring components during septation and define the site of cytokinesis. They also recruit a toroidal cortical F-actin network to the appressorium pore that provides cortical rigidity to facilitate plant infection. Septins act as diffusion barriers for proteins that mediate membrane curvature necessary for penetration peg formation. Repolarization of the F-actin cytoskeleton at the appressorium pore is essential for plant penetration and is controlled by cell polarity regulators, such as Cdc42 and Chm1. Septin-mediated plant infection is regulated by NADPH oxidase (Nox) dependent generation of reactive oxygen species (ROS). The Nox2/NoxR complex is essential for septin organization at the appressorium pore. Septins are therefore key determinants of appressorium repolarization. I also report an investigation of fungal secretory processes during tissue invasion and present evidence that distinct pathways are involved in effector secretion by Magnaporthe oryzae. A BrefeldinA-sensitive pathway is necessary for secretion of apoplastic effectors, such as Bas4 and Slp1, while a BrefeldinA-insensitive pathway is necessary for secretion of effectors destined for delivery to rice cells.The Halpin Scholarship for Rice Blast Researc

NADPH oxidases regulate septin-mediated cytoskeletal remodeling during plant infection by the rice blast fungus

notes: PMCID: PMC3581893types: Journal Article; Research Support, Non-U.S. Gov'tThe rice blast fungus Magnaporthe oryzae infects plants with a specialized cell called an appressorium, which uses turgor to drive a rigid penetration peg through the rice leaf cuticle. Here, we show that NADPH oxidases (Nox) are necessary for septin-mediated reorientation of the F-actin cytoskeleton to facilitate cuticle rupture and plant cell invasion. We report that the Nox2-NoxR complex spatially organizes a heteroligomeric septin ring at the appressorium pore, required for assembly of a toroidal F-actin network at the point of penetration peg emergence. Maintenance of the cortical F-actin network during plant infection independently requires Nox1, a second NADPH oxidase, which is necessary for penetration hypha elongation. Organization of F-actin in appressoria is disrupted by application of antioxidants, whereas latrunculin-mediated depolymerization of appressorial F-actin is competitively inhibited by reactive oxygen species, providing evidence that regulated synthesis of reactive oxygen species by fungal NADPH oxidases directly controls septin and F-actin dynamics.Biotechnology and Biological Sciences Research Council (BBSRC)National Natural Science Foundation of ChinaHalpin ScholarshipEuropean Research Council Advanced Investigator Awar

A new ZTL-type F-box functions as a positive regulator in disease resistance: VIGS analysis in barley against powdery mildew

In our previous studies, we found a new Zeitlupe (ZTL) type F-box protein which is expressed at a higher level upon avirulent pathogen infection (Bozkurt et al., 2007). F-box proteins mark the proteins to be degraded through 26S proteasome system by ubiquitination. Since the information on the role of ubiquitin mediated proteolysis in disease responses is advancing rapidly, we sought to understand the way which F-box functions in resistance response as part of ubiquitin-proteasome pathway. Interestingly, in response to silencing of this F-box gene via BSMV mediated virus induced gene silencing (VIGS) method, barley plants lost resistance towards avirulent pathogen race. The Pallas-01 line having Mla1 R-gene showed hyphae formations when inoculated with avirulent powdery mildew race, Bgh103, after 4-fold silencing. This observation suggests that F-box protein functions as a positive regulator in powdery mildew disease mechanism and broadens the function of ZTL-type F-box proteins, previously known to have roles only in circadian clocks, flowering time control, and phytochrome pathway

Septin-mediated plant cell invasion by the rice blast fungus, Magnaporthe oryzae

Author's accepted version, please cite the published version by following the DOI link.To cause rice blast disease, the fungus Magnaporthe oryzae develops a pressurized dome-shaped cell called an appressorium, which physically ruptures the leaf cuticle to gain entry to plant tissue. Here, we report that a toroidal F-actin network assembles in the appressorium by means of four septin guanosine triphosphatases, which polymerize into a dynamic, hetero-oligomeric ring. Septins scaffold F-actin, via the ezrin-radixin-moesin protein Tea1, and phosphatidylinositide interactions at the appressorium plasma membrane. The septin ring assembles in a Cdc42- and Chm1-dependent manner and forms a diffusion barrier to localize the inverse-bin-amphiphysin-RVS-domain protein Rvs167 and the Wiskott-Aldrich syndrome protein Las17 at the point of penetration. Septins thereby provide the cortical rigidity and membrane curvature necessary for protrusion of a rigid penetration peg to breach the leaf surface.Biotechnology and Biological Sciences Research Council (BBSRC)ER